A critical parameter of certain internal combustion engines such as diesel engines, is the presence of any gas (such as combustion product or air) in the coolant of the cooling system. This is particularly true in the case of engines which have cylinder sleeves that are designed for operation at high pressures and therefore high temperatures, as is the case in most diesel engines. A small amount of gas in the coolant can be catastrophic due to the fact that the bubbles of gas have a tendency to collect on the outside surfaces of the cylinder sleeves, thereby providing an insulation barrier between the coolant and the sleeve. As a consequence, sleeve hot spots can result, which when carried to the extreme, cause a breach in the engine which can totally wreck it.
A further problem is the fact that this condition can exist without manifesting any indication thereof in the normal cooling system thermometer in the cab of the vehicle in which the engine is mounted. Even though very large leaks in head gaskets and the like can result in coolant being drawn into the engine, and manifest itself in either the quality or quantity of the engine oil, smaller leaks, incapable of allowing liquid to pass therethrough on a relatively low pressure suction stroke, can nonetheless allow combustion gases to pass therethrough into the coolant as a consequence of the much higher pressure strokes.
Since the seriousness of this condition is well known, a test has previously been used to determine the presence of excessive gas in the coolant of an engine. This test provides a radiator cap having a hose attached thereto, the hose being passed loosely into the neck of a bottle full of water which is standing upside down with its opening below the level of water in a bucket. The time required to displace all of the water in the bottle is utilized as a measure of the amount of gas in the coolant. The problem with this test is that it is cumbersome and somewhat difficult to perform with ordinary buckets and bottles. A further problem is, because of the undesirability of performing such a test, the test is rarely performed; in fact, the failure to perform any such test leaves many vehicles found wanting for minor repair of head gasket leaks, prior to destruction of the engine.